If B says to do it, then you should do it. Make sure he knows that in order to do A's requests, you need to push back some other tasks. Let him prioritize your tasks.
3. -- 覺得是不太可能和A談了。怎麽談?說他因為我換組故意給我穿小鞋?
No, simply tell him "Please don't yell at me" when he's doing it.